1. Field of the Invention
This invention relates to a thermoplastic resin composition having excellent impact resistance, weather resistance and moldability.
2. Description of the Prior Art
ABS resin is a thermoplastic resin having a well-balanced combination of impact resistance, thermal and mechanical properties, and moldability, and has conventionally been used in wide applications including electrical parts, automobile parts and other miscellaneous goods. However, it is well known that ABS resin is subject to deterioration by ultraviolet light or the like and hence exhibits poor weather resistance, because the butadiene-based polymer constituting its rubber component for imparting impact resistance to the resin has a large number of chemically unstable double bonds in the backbone.
In order to improve the weather resistance of ABS resin, there have been proposed a number of processes for the preparation of an impact-resistant resin by using a saturated rubbery polymer having few double bonds in the backbone. Typical examples of such processes are those using an ethylene-propylene-non-conjugated diene terpolymer rubber.
These processes for the preparation of an impact-resistant resin include a bulk polymerization process (as disclosed in U.S. Pat. No. 3,435,096), solution polymerization processes (as disclosed in U.S. Pat. Nos. 3,538,190 and 3,538,191) and the like. Moreover, there have been proposed emulsion polymerization processes in which an ethylene-propylene-non-conjugated diene terpolymer rubber is emulsified, partially crosslinked and then subjected to graft polymerization (Japanese Patent Publication Nos. 16394/'68 and 35718/'73).
In Japanese Patent Publication No. 16394/'68, it is stated that the impact strength of the resulting resin is markedly improved by cross-linking a part of the ethylene-propylene-non-conjugated diene terpolymer latex. However, when the degree of cross-linking (i.e., the gel content) is at such a low level as described in the examples of that patent, the resulting resin exhibits good impact strength but fails to present an attractive molding appearance.
In Japanese Patent Publication No. 35718/'73, it has been proposed to improve polymerization stability by carrying out the graft polymerization in the presence of a hydrocarbon. This proposition has been made because the ethylene-propylene-non-conjugated diene terpolymer latex, which is emulsified by mechanical means, tends to have poor stability. However, the ethylene-propylene-non-conjugated diene terpolymer rubber used in the examples of that patent has a low degree of cross-linking and the resulting resin fails to present an attractive molding appearance.
Thus, graft copolymers obtained by the processes described in the above-referenced patents have better weather resistance than ABS resin, because an ethylene-propylene-non-conjugated diene terpolymer rubber is used as the rubber component. However, the external appearance and surface gloss of molded articles formed therefrom are still unsatisfactory. In the existing state of the art, therefore, a thermoplastic resin having a desired combination of excellent weather resistance and attractive molding appearance is not yet available.